Manchester coding is used to transmit signals from a transmitter to a receiver. As a rule, the transmission data stream is in the form of a data signal and a clock signal. In order to avoid the transmission of two signals from the transmitter to the receiver, a Manchester-encoded signal is generated from the clock signal and the data signal. It is therefore possible to transmit the data stream over one shared signal line to the receiver. However, since the receiver likewise needs a clock signal and a data signal again for further processing, the Manchester-encoded signal must be decoded again. This decoding requires the recovery of the clock signal from the signal transmitted.
In the decoding methods known from the related art, in the simplest case, the receiver synchronizes itself to the first start bit received, and in each instance evaluates all following bits at half the a priori known bit time. To also permit the correct readout of the last bit, the time base of the receiver after the last transmitted bit is allowed to be deviated by the half-bit time at most. Thus, the Manchester-encoded signal must be sampled with a frequency which, for typical application cases, is allowed to deviate only by 0.2% from the transmit frequency. This requirement can only be satisfied with great expenditure, and in any event, in a manner relatively susceptible to faults.
Therefore, known decoding methods rely on phase-locked loops (PLL) which, on the basis of the signal edges occurring in the input signal, continuously track the sampling frequency of the receiver in such a way that it is kept as synchronous as possible with the transmit frequency. In this way, the clock signal of the transmitter is reconstructed. However, disadvantages of methods which generate a synchronize clock signal for the sampling of the input signal are that interferences in the input signal may fall at the sampling instant and therefore cause errors in the decoding. An example for the one-time sampling of the input signal within a half-bit time is found in U.S. Pat. No. 4,881,059 A.
A further disadvantage of the PLL is that it is a feedback structure or structure controlled in closed-loop which must pass through a transient time up to the desired function, and whose interpretation of the control parameters must be optimized with respect to possible instabilities. Especially in the case of short messages, which in each case are transmitted with a time interruption, the result is that a new transient phase is produced in each instance. Thus, there is the possibility that the beginning of each message cannot be decoded correctly.
The U.S. Pat. No. 7,133,482 B2 describes a method which is intended to avoid the disadvantages described. In that case, a two-step signal evaluation is carried out. In a first step, the received signal is filtered, so that interferences cannot negatively influence the downstream evaluation process. The evaluation is carried out downstream by evaluating the filtered input signal at fixed points in time after the last recognized edge, thereby permitting a continuous adjustment to the clock pulse of the transmitter without a PLL. A range-limited integrator is used as input filter.
In U.S. Pat. No. 5,696,800, a method is described in which fault tolerance is achieved by the generation of two clock signals that are produced by filters of different bandwidths. However, costly structures are thereby obtained.
In U.S. Pat. No. 6,370,212 B1, the use of a moving-type averaging filter is introduced for evaluating the Manchester-encoded signal. This implementation generates the decoded clock signal, but still from a phase-locked loop downstream of the moving-type averaging filter. Because of this, an incoming signal may first be evaluated as of the instant from which the phase-locked loop is also synchronized. Therefore, the complete decoding of short signal sequences is not possible. In addition, transition points are generated as input signal for the phase-locked loop. Since this signal is derived from the first slope change, in the case of disturbed input signals, marked time deviations may result which hamper the decision as to whether the phase-locked loop is locked in and consequently the decoded signals are valid. Therefore, only limited interference insusceptibility is achieved.